Electrooptical system



NYQUIST 1,72

ELECTROOPTICAL SYSTEM Filed Aug. 22, 1933 VVVV INVENTO/P H. NFL I57ATTORNEY Patented Feb. 25, 1936 UNITED STATES ELECTROOPTICAL SYSTEMHarry Nyquist, Millburn, N. J assignor to American Telephone andTelegraph Company, a corporation of New York Application August 22,1933, Serial No. 686,306

7 Claims.

The present invention relates to electro-optical systems and moreparticularly to receiving means for use in television.

One form of present day television receivers comprises an optical systemincluding a light source supplying light of constant intensity which iscontrolled by means actuated by the incoming image current to vary thelight supplied to the eye of an observer, or projected on a screen, in

10 accordance with the tone values of successive elemental areas of thefield of view scanned at a transmitter, or a light source supplied withreceived image current which causes the intensity of the light suppliedby it to vary as the tone values of the successive elemental areas ofthe scanned field, in combination with a moving member provided with aseries of apertures arranged in a spiral, operated in synchronism and inphase with the scanning member of the transmitter and at a rate suchthat the complete scanning cycle is efiected within the period ofpersistence of vision, whereby the light of varying intensity isutilized to set up on the retina of the eye of an observer, or on areceiving screen, an

image of the field of View scanned at the transmitter.

For high quality image production, the image current necessarilyconsists of a band of current components which extend from approximatelyzero frequency to an upper limiting frequency of several thousandcycles. In the operation of such image producers, wherein a light sourceis supplied with image current through amplifiers, a certain amount ofdistortion is introduced,

caused by the non-linear relation between current supplied. to the lampand the brightness of the light, i. e., due to the failure of the lampto faithfully follow the current variations, and the sensitivity of thelamp and image current amplifiers to variations of the energizingvoltages supplied to them.

As the art develops and increased definition is resorted to, the upperlimiting frequency of the image current may well be of the order of amil-' 5 lion cycles, in which event the distortion due to the abovementioned causes may be greater than can be tolerated and may be sogreat that it will be impossible to produce an intelligible image.

An object of the present invention is to overcome the above noteddisadvantages by supplying the signal variations through a balancedbridge network, one arm of which is connected to a light source, usingradiations from the light source to 55 control the production of currentvariations and apply these variations to the bridge so that they are inphase opposition to signal variations supplied to the light source.

A detail description of the invention follows 60 and is illustrated onthe attached drawing, which is a diagrammatic showing of one form oftelevision receiver involving the invention.

A television transmitting apparatus of the type disclosed in applicationof Frank Gray, Serial No.

227,649, filed October 21, 1927, may be used to 5 supply an imagecurrent, varying as the tone values of elemental areas of the field ofview or object at the transmitting station, for transmission over achannel, 1. e., either a wire line, or a line carrier or radio channel.10

Such transmitting apparatus is diagrammatically shown in the drawing ascomprising an optical system including a source S supplying light ofconstant intensity, a lens b, a disc is, driven by a motor M andprovided with apertures a ar- 15 ranged in a spiral for producing amoving beam of light of high intensity for illuminating successiveelemental areas of an object or field of view F, and a light sensitivedevice P, adapted to receive light reflected by the object to be thereby2o activated to cause the production of an image current, which isamplified by the device A and supplied to a transmission circuit TC.This circuit may be connected to a transmission line or to the input ofa carrier transmitting apparatus, 25 the output circuit of which may beconnected to a carrier line or radio transmitter. The field of view iscompletely scanned during one revolution of the disc 10, which should berotated at such speed that complete scanning is effected within 30 theperiod of persistence of vision. For a more detailed disclosure of thetransmitting apparatus, reference may be made to the above-mentionedGray application.

At the receiving station the incoming image 35 current is received bythe device R. For direct transmission over a wire line, this device mayinclude a selecting and amplifying means and, for carrier transmission,the device B may include detecting or demodulating means as well as the40 necessary selecting and amplifying means. In either case the imagecurrent is supplied through the circuit I connected to one set ofconjugate terminals 2 and 3 of the conjugate network or Wheatstonebridge 4. The signal potentials developed across the arm 2-5 areimpressed upon the input electrodes 1 and 8 of the amplifier 9, theoutput circuit of which includes the glow lamp I0. Associated with theglow lamp I0 is a rotating disc I I provided with a spiral of aperturesi2. This disc is operated to effect a complete revolution within theperiod of persistence of vision, in synchronism and in phase with thescanning member at the transmitter. It thereby cooperates with the lightsource ID to set up an image of the distant field of View or object,which may be directly viewed by an observer D or projected on a screenby means of a suitable optical system.

Any suitable means may be used for maintaining the discs R: and II insynchronism and in phase with each other. For example, the synchronizingsystem disclosed in U. S. Patent 1,999,376 of H. M. Stoller, issuedApril 30, 1935, may be used for this purpose.

In the field of radiation from the lamp I is a light sensitive device l3herein shown as a photoelectric cell adapted to be energized by a sourceH1 in series with a resistance I 5. This resistance is connected to theinput electrodes of a vacuum tube amplifier l6 which is supplied withspace current by the source l1.

The output circuit I8 of the amplifier I6 is connected through avariable attenuator [9 to terminals 6 of the conjugate network 4. Byadjusting the attenuator I9, the amplitude of the current supplied tothe network may be controlled and, for the purpose of the presentinvention, the current supplied to the terminals 56 should be of suchsign that the current flowing through the arm 2-5 of the network is inphase opposition to the signal current supplied over the line I.

As is now well known, the use of glow lamps in connection with thetransmission of pictures for television and telephotography has beencharacterized by a number of disadvantages. First, the relation betweeninput current and the brightness of the light supplied by the lamp isnonlinear. This non-linearity effect causes the contrast in certain toneregions to be compressed and in other regions to be extended. Again, thelamp does not always faithfully follow the current variations, and boththe lamp and its attendant amplifier are very sensitive to variations inthe supply voltages, so that allowable changes in these voltages must berestricted within very narrow limits in order to obtain satisfactoryoperation of the reproducing apparatus. Moreover, vacuum tubeamplifiers, used as repeaters, have been found to suffer fromnon-linearity of response, and sometimes fail to faithfully followvariations of the voltages impressed upon their input electrodes. All ofthese effects tend to cause distortion of the picture or image producedat the receiver with respect to the original scanned at the transmitter.

In the operation of the apparatus described above, the light supplied bythe lamp It) not only varies in accordance with the varying amplitudesof the signal current, but also in accordance with the variationsintroduced by the undesired or distorting operation of the lamp andamplifiers, etc. This fluctuating light is supplied to the photoelectriccell l3 which is thereby activated to cause the production of a currentwhich varies as the signals and also in accordance with the undesiredvariations. This current after being amplified by the device l6 andregulated by the attenuator I9 is fed back to the input electrodes ofthe amplifier 9 in such manner that its direct current component opposesthe direct current component of the signal current supplied by circuitor line I to reduce the gain of amplifier 9 and thereby render it morelinear in its response and less susceptible to variations of its supplysource. A further improvement is obtained, in that by maintaining theloss through the feedback circuit distortionless with respect tofrequency for the frequency region of amplification of the repeater, theoverall gain of the repeater will be made more nearly constant in thatregion. These improvements will all increase in proportion to the extentto which the overall gain is reduced by the energy fed back. Forpractical purposes, it is desired to keep the suppression effected bythe energy fed back independent, at least within reasonable limits, ofthe impedances of the signal input and output circuits connected to therepeater or amplifier 9. For a more complete disclosure of the gainsuppression amplifier, briefly described above, reference may be made toapplication, Serial No. 606,871 of H. S. Black, filed April 22, 1932.

While for purposes of illustration, a single stage of amplificationincluding one vacuum tube 9 is shown for repeating the signal and butone repeating tube I6 is shown in the feedback circuit, it is to beunderstood that more than one tube may be used in either or both ofthese circuits. However, it is essential that the number of repeatertubes in the respective circuits and their connections be such that thephotoelectric current produced by the cell l3 shall be applied to theinput electrodes of repeater 9 in such phase relation as to oppose thesignal wave supplied thereto over the circuit I. Obviously if a twostageamplifier were used in place of tube 9, it would be necessary to eitherreverse the connections of the photoelectric cell [3 to amplifier [6 orto use two stages of amplification in place of the single stage shown atIS, in order to preserve the correct phase relation between the signaland the energy feedback to the input electrode of the two stageamplifier.

In the arrangement illustrated it is only necessary to use a singleconjugate network, 1. e., in connection with the input side of theamplifier 9, inasmuch as reasonable precautions can readily be taken tomake the optical coupling between the lamp [0 and photoelectric cell l3independent of the conditions existing on the observers side of thescanning disc. However, if desired, a conjugate network may be includedin the output circuit of amplifier 9, as disclosed in the abovementioned Black application.

If it is desired to prevent the circulation of direct current in theconjugate network and if the received signal contains no useful directcurrent component, blocking condensers may be inserted in appropriateplaces in the input bridge.

The system described above possesses the following advantages. Themodulation products and harmonics produced by the repeater 9 are reducedin proportion to the amount of gain suppression, the susceptibility ofthe lamp to variations in its voltage supply and in the voltage suppliedto the amplifier 9 is reduced in proportion to the amount of gainsuppression and the speed of response of the glow lamp is increased, aswill be apparent from the following consideraion.

When gain suppression is not used, the lag of the lamp may be viewed asa diminution of the response of the lamp with increase of frequency ofthe applied signal wave. That is, for a given current of pure sine wavesupplied to the receiver, the brightness of the glow will decrease asthe frequency is increased. However, if the feedback circuit itself isdistcrtionless with frequency, the use of gain suppression will make theoverall characteristic of the apparatus more nearly distortionless, withthe result that a given diminution of brightness in response of the lampfrom that given for a corresponding direct current applied thereto, willnow be reached at a higher frequency than would be the case if thefeedback for reducing the gain of the repeater were not used.

What is claimed is:

1. An electro-optical receiver comprising a conjugate network, means forsupplying incoming image current to one set of conjugate terminals ofsaid network, an amplifier supplied with image current variationsproduced in one arm ofsaid network, a light source included in theoutput circuit of said amplifier, means for scanning light supplied bysaid source to control the production of an image of a field of view,and means for causing the response of said light source to be linearlyrelated to said incoming image current, comprising a light sensitivemeans supplied with light from said source, means for supplying currentvariations produced by said light sensitive means to the other conjugateterminals of said network, said last-mentioned means including means forcausing the varying current flowing through the image current arm ofsaid network and supplied to said amplifier to be in phase opposition tothe incoming image current supplied thereto.

2. An electro-optical receiver comprising a Wheatstone bridge, means forsupplying incoming image current to one diagonal of said bridge, anamplifier supplied with image current variations produced in one arm ofsaid bridge, a light source included in the output circuit of saidamplifier, means for scanninglight supplied by said source to controlthe production of an image of a field of view, and means for causing theresponse of said light source to be linearly related to said incomingimage current, comprising a light sensitive means, activated by lightfrom said source to control the production of current varying as thelight variations supplied by said source, means for supplying saidvarying current to the other diagonal of said bridge, saidlast-mentioned means including means for causing the varying current,flowing through the image current arm of the bridge and supplied to saidamplifier, to be in phase opposition to the incoming image currentsupplied thereto.

3. An electro-optical receiver comprising a Wheatstone bridge, means forsupplying incoming image current to one diagonal of said bridge, anamplifier supplied with image current variations produced in one arm ofsaid bridge, a light source included in the output circuit of saidamplifier, a moving member having apertures for scanning the lightsuppliedby said source to control the production of an image of a fieldof view, and means for causing the response of said light source to belinearly related to said incoming image current, comprising a lightsensitive electric means activated by light from said source to controlthe production of a varying current, means for supplying said varyingcurrent to the other diagonal of said bridge, said last-mentioned meansincluding means for causing thev image current, flowing through theimage current arm of said k3 ridge and supplied to said amplifier, to bein phase opposition to the incoming image current supplied thereto, andan attenuator for controlling the value of the varying current suppliedto said bridge.

4. An electro-optical receiver comprising a conjugate network, means forsupplying signal current corresponding to a picture or representation toone set of conjugate terminals of said network, an amplifier suppliedwith signal current variations produced in one arm of said network, alight source included in the output circuit of said amplifier, means forutilizing light from said source to control the production of an imageof the picture or representation, and means for causing said source tofaithfully respond to'variations of said signal current, comprising alight sensitive means supplied with light from said source, means forsupplying current variations produced by said light sensitive means tothe other conjugate terminals of said network, and means, included insaid last-mentioned means, for causing said current variations flowingthrough the signal current arm of said network to be in phase oppositionto the incoming signal current supplied thereto.

5. An electro-optical receiver comprising a conjugate network, means forsupplying signal current corresponding to a picture or representation toone set of conjugate terminals of said network, an amplifier suppliedwith signal current variations produced in one arm of said network, alight source included in the output circuit of said amplifier, means forutilizing light from said source to control the production of an imageof the picture or representation, and means for increasing the speed ofresponse of said source comprising a light sensitive means supplied withlight from said source, means for supplying current variations producedby said light sensitive means to the other conjugate terminals of saidnetwork, and means, included in said last-mentioned means, for causingsaid current variations to be in opposite phase to signal currentflowing through the signal current arm of said network.

6. An electro-optical receiver comprising a conjugate network, means forsupplying signal current corresponding to a picture or representation toone set of conjugate terminals of said network, an amplifier suppliedwith signal current variations produced in one arm of said network, alight source included in the output circuit of said amplifier, means forutilizing light from said source to control the production of an imageof the picture or representation, and means for increasing the range ofsignal frequencies to which the source will faithfully respond,comprising a light sensitive means supplied with light from said source,means for supplying current variations produced by said light sensitivemeans to the other conjugate terminals of said network, and means,included in said last-mentioned means, for causing said currentvariations to be in opposite phase to signal current flowing through thesignal current arm of said network.

7. An electro-optical receiver comprising a conjugate network, means forsupplying signal current corresponding to a picture or representation toone set of conjugate terminals of said network, an amplifier suppliedwith signal current variations produced in one arm of said network, alight source included in the output circuit of said amplifier, means forutilizing light from said source to control the production of an imageof the picture or representation, and means for rendering the responseof said light source substantially constant in the frequency range ofsaid signal current comprising a feed-back circuit which isdistortionless in the frequency range of said signal current andincludes means for causing the current fed back to be in phaseopposition to the signal current flowing through the said one arm of thenetwork.

HARRY NYQUIST.

